Hooks, Lines, and Sinkers: How AGN Feedback and Cosmic-Ray Transport shape the Far Infrared-Radio Correlation of Galaxies

TL;DR

This study uses advanced cosmological simulations with cosmic-ray physics and AGN feedback to reproduce and analyze the far-infrared-radio correlation in galaxies, revealing insights into cosmic-ray transport and galaxy evolution.

Contribution

First to reproduce the z~0 FRC using detailed simulations with explicit cosmic-ray spectra and multiple feedback models, exploring their effects on galaxy observables.

Findings

FRC predictions align with calorimeter theories at high FIR luminosity.

Increasing UV and CRe escape explains the FRC conspiracy at low FIR luminosity.

AGN feedback influences outliers, with radio-excess objects linked to IR-dim, quenching galaxies.

Abstract

The far-infrared (FIR) - radio correlation (FRC) is one of the most promising empirical constraints on the role of cosmic-rays (CRs) and magnetic fields (\textbf{B}) in galaxy formation and evolution. While many theories have been proposed in order to explain the emergence and maintenance of the FRC across a gamut of galaxy properties and redshift, the non-linear physics at play remain unexplored in full complexity and cosmological context. We present the first reproduction of the $z \sim 0$ FRC using detailed synthetic observations of state-of-the-art cosmological zoom-in simulations from the FIRE-3 suite with explicitly-evolved CR proton and electron (CRe) spectra, for three models for CR transport and multi-channel AGN feedback. In doing so, we generally verify the predictions of `calorimeter' theories at high FIR luminosities (\Lsixty\, $\gtrsim$ 10$^{9.5}$) and at low FIR luminosities (\Lsixty\, $\lesssim$ 10$^{9.5}$) the so-called `conspiracy' of increasing ultraviolet radiation escape in tandem with increasing CRe escape, and find that the global FRC is insensitive to \textit{orders-of-magnitude} locally-variable CR transport coefficients. Importantly, the indirect effect of AGN feedback on emergent observables highlights novel interpretations of outliers in the FRC. In particular, we find that in many cases, `radio-excess' objects can be better understood as \textit{IR-dim} objects with longer-lived radio contributions at low $z$ from Type Ia SNe and intermittent black hole accretion in quenching galaxies, though this is sensitive to the interplay of CR transport and AGN feedback physics. This creates characteristic evolutionary tracks leading to the $z=0$ FRC, which shape the subsequent late-time behavior of each model.